Power MOSFETs (metal oxide semiconductor field effect transistor) are examples of high breakdown voltage semiconductor devices which are used for switching power sources, inverter devices or the like. For example, these MOSFETs are configured to switch high voltages at low-ohmic loads so as to have a very small switching and conduction loss and thus requiring a small on resistance (Ron) and a high breakdown voltage when being switched off. For example, a power MOSFET should withstand a drain to source voltage VDS of some tens to some hundreds Volts when being switched off. As a further example, power MOSFETs conduct a very large current which may be up to some hundreds of Amperes at a gate-source voltage of about 10 to 20 V at a low voltage drop VDS.
In order to meet the increasing demands on small Ron and high breakdown voltage, it is desirable to develop new concepts of a semiconductor device, e.g. a vertical semiconductor device.